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Copper Finishes versus Layer Thickness

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#46436

Copper Finishes versus Layer Thickness | 2 January, 2007

I have a new customer that has a print calling for 2oz. copper finish for top and bottom layers, and 1oz. copper finish for the inner 6 layers. Now, they also have a stack up depicting each layer and thickness. It shows the outer layers of copper being 0.7 mils thick with 0.7 mil plating. Inner layers are 1.4 mils thick. Substrate is 8mill, 7.2mill and 5 mil. Now I follow the stack up and can see they add up to about an 0.063" board.

How does copper finish and the stack-up relate mathematically?

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#46476

Copper Finishes versus Layer Thickness | 4 January, 2007

You cannot determine foil thickness from its weight per area, because of the significant variation in the density of electrodeposited [ED] copper.

But people don't care. They do it regardless. (oz/ft2)||(mils)||(mm) 1/2||0.7||17.8 1||1.4||35.6 2||2.8||71.1 3||4.2||106.7

The thickness of 1oz Cu foil may start out at 1.4 thou but it ends up at about 1.3 thou due to processing steps such as deburring and microetch. So in the real world, the boards that were fabricated using 1 oz foil should have a FINISHED Cu foil thickness of 1.3 thou after conventional processing.

To complicate this further, it's very uncommon to see 1 oz copper at 1.3 thou in a finished board. Almost everyone uses foil that is at the thin end of the spec. Why? Because copper is sold by weight and thinner foil weighs and costs less. IPC 4562 says that ED foils can be 10% below the listed nominal (nominal for 1 oz is 34.3 um or 1.35 thou). Foil more commonly comes in at 1.30 thou or less. Processing does reduce this down to 1.2 or thinner.

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#46488

Copper Finishes versus Layer Thickness | 4 January, 2007

Hi Chunks,

Starting copper on outer layer with .0007 is 1/2 oz. copper foil. so if starting copper is .0007 and your Board house will plate an additional .0021 mils of copper. which will give you a finished copper weight of .0028 Mils or (2 oz.)

Your Internal copper again starting at 1.4 mils will be around 1.38 -1.39 after Innerlayer etch.

A standard 6 layer stack-up to hit .062 +/-.007 would be.

1/2 copper foil .007 mils pre-pre (1 sheet of 7628) .015 1/1 Core material (1/1 = 1 oz. copper on each side) .007 mils pre-pre (1 sheet of 7628) .015 1/1 Core material (1/1 = 1 oz. copper on each side) .007 mils pre-pre (1 sheet of 7628) 1/2 copper foil Thickness after laminating would be .0552 then add plating and solder mask and surface finish. Final thickness would be .061 - .062

Hope this helps,

Mike

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